I. Field
The present disclosure relates generally to communication, and more specifically to techniques for performing channel estimation.
II. Background
A wireless multiple-access system may include Node Bs (or base stations) that communicate with user equipments (UEs). Each UE may communicate with one or more Node Bs via transmissions on the downlink and uplink. The downlink (or forward link) refers to the communication link from the Node Bs to the UEs, and the uplink (or reverse link) refers to the communication link from the UEs to the Node Bs.
The system may support multiple input multiple-output (MIMO) transmission on the downlink and/or uplink. On the uplink, one or more UEs may send transmissions from multiple (T) transmit antennas to multiple (R) receive antennas at a Node B. A MIMO channel formed by the T transmit antennas and R receive antennas may be decomposed into C spatial channels, where C≦min {T, R}. Improved performance (e.g., higher throughput and/or greater reliability) may be achieved by exploiting the spatial channels formed by the multiple transmit and receive antennas.
Any number of UEs may desire to transmit data on the uplink to a Node B at any given moment. Good performance may be achieved by selecting one or more “compatible” UEs whose transmissions will interference with one another as little as possible at the Node B. Compatibility may be determined by (i) estimating the channel response from each transmit antenna for each UE desiring to transmit data on the uplink to each receive antennas at the Node B and (ii) selecting a set of transmit antennas with channel responses that are most orthogonal to one another. The channel response for each transmit antenna may be estimated based on a pilot sent from that transmit antenna. Each UE desiring to transmit data on the uplink may be allocated radio resources to transmit pilot from each antenna at that UE. However, a large amount of uplink resources may be consumed for uplink pilots used for channel estimation to select compatible UEs for MIMO transmission on the uplink.
There is therefore a need in the art for techniques to efficiently estimate uplink channel responses for UEs without consuming excessive uplink resources.